Conventionally, carrier and toner are used for a two component developing method which is a representative developing method of the electrostatic image developing method. Now, a coating carrier, in which a magnetic particle is coated with resin, is commonly used as a practical carrier.
As resins used for coating, there are a large number of resins such as styrene/acrylic acid ester types, fluorine types, silicone types, and the like. Polyolefin type resins may also be used as one of these resins. Polyolefine type resins have the following advantages: water repellency is strong; a thick film can be applied with increared mechanical elasticity; stresses applied onto the carrier can be absorbed and decreased; and adherence onto toner is minimized.
For example, a carrier, in which magnetic particles are fusion-coated by a polypropylene resin, is disclosed in Japanese Patent Publication Open to Public Inspection No. 154639/1977. Further, a coated carrier in which magnetic particles are mechanically coated by a polytetrafluoroethylene resin, is disclosed in Japanese Patent Publication Open to Public Inspection No. 35735/1979. However, in these carriers, the adhesive property to the core magnetic particle is insufficient, so that unacceptable film peeling occurs and these carriers have insufficient durability for a long period of time, which is a major problem.
As a countermeasure to the problem, a coated carrier, coated by a surface polymerization coating method of polyolefine is disclosed in Japanese Patent Publication Open to Public Inspection No. 106808/1985, and further, another coated carrier, which is surface-polymerization-coated with a polyethylene resin, is disclosed in Japanese Patent Publication Open to Public Inspection No. 187770/1990. However, these carriers cause unevenness in catalyst carrying positions, so that nonuniform coating conditions occurs on the carrier surface. Therefore, lowered fluidity of the carrier, nonuniform charging amounts, toner-spent, and charge-up occur. Accordingly, although the carrier has the above-described advantages, it has a major practical problem which has not yet been overcome.
As a carrier for use in electrostatic image developers for electrophotography, or the like, a carrier, which is made such that polyolefine is polymerized onto the surface of magnet particles such as ferrite, or the like, and therewith the surface of the particle is covered, is proposed in Japanese Patent Publication Open to Public Inspection No. 187770/1990. In this proposal, although a good carrier is obtained in which a toner-spent or durability of carrier covered resin is greatly improved, there is a problem, in which magnetic particles are exposed on a portion of the carrier surface, or a possibility in which polymerization is unstable depending on surface metals of the magnetic particles. On such the exposed portions of magnetic particles, toner tends to be spent especially under conditions of high temperature and high humidity. When polymerization is unstable, adhesive strength between polyolefine and magnetic particles is weak, and therefore coating film peeling occurs, especially under low temperature and low humidity conditions. Further, the specific gravity of ferrite is not more than that of iron powder, but comparatively not so smaller, and therefore, there is a problem in which the charging property varies due to stress in the developing unit, which is still under investigation.
In order to decrease the specific weight of the carrier, a carrier is proposed in which polyolefine is polymerized onto the surface of a binder type core on which magnetic particles are dispersed, as disclosed in Japanese Patent Publication Open to Public Inspection No. 70853/1992. Although the binder type carrier has a lesser specific weight of 2.0 through 3.0, and is effective in decreasing stress in the developing unit, the surface of the binder type core is relatively smooth, and thereby, catalyst tends to be barely carried. Therefore, the cover due to polymerization of the polyolefine is not uniform, so that the adhesive strength between the core and cover resin is relatively weak, which is a problem.
An object of the present invention is to solve the above-described problems while inherent features of polyolefin resin, that is, strong repellency, relatively high mechanical strength due to large film thickness, stresses applied onto the carrier being absorbed or lightened, and further, toner fusion-adherence rarely occurring, etc., are being maintained.
The present invention is made under the above-described conditions, and an object of the present invention is to provide a carrier for use in an electrostatic image developer in which no toner-spent or non-film peeling occur even under a long period of use, and the charging property does not vary, so that high image quality can be constantly maintained.
Therefore, an object of the present invention is to provide a carrier by which an outputted image, with maximum image density and high resolution, can be stably maintained over an extended period of time.